Before Betty catches the taxi, the glass reflection shows a taxi with a blue roofplate

While Betty is talking, the plate is blurred out. But the third letter doesn't seem 6 to me. somehow it seems A

I don't live in the US. In my mind most cabs have ads on the roof. Is it common to have a alphanumeric plate on it? What does that signify?
Does 1X63 carry any meaning?

The one in which Betty and Rita visit Sierra Bonita says "TAXI"